The flash player does the unbelievably brain dead thing of checking the glxinfo vendor strings to make sure they do *not* contain the string "SGI". Flash hardware acceleration will only be enabled if the vendor strings do not contain the string "SGI". Incredible, huh? I guess this is the kind of idiocy we've got to contend with in closed-source binary blobs.

The advice given in one of the comments actually worked for me. Specifically, use a hex editor (such as 'ghex') to alter these two binaries:

/usr/lib/libGL.so.1.2
/usr/lib/xorg/modules/extensions/libglx.so

(or the actual binaries these files link to, if they are symbolc links. They were links on my system.)

They each contain one instance of "<null>SGI<null>" (plenty of the string "SGI" in other contexts, but ignore these). Change the single instance in each file to "<null>ATI<null>". Restart X, and you may just have hardware accellerated flash. It worked for me.

I tried this method on my Samsung NC10 (poor Intel graphics) and it did a magic, still thou i can say for sure, that Adobe flash plug-in for Firefox sucks, and if it is Firefox + *nix = sucks more. So blame Adobe, they think that everybody uses M$ IE.